This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-100536, filed Mar. 30, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention generally relates to the field of disk drives, and more particularly, to a method and an apparatus for controlling a spindle motor for rotating a disk.
2. Description of the Related Art
Generally, in a disk drive that may typically be a hard disk drive, the disk that operates as data recording medium is fitted to a spindle motor (SPM) and driven to rotate. In recent years, 3-phase brushless DC motors of the sensorless type are in the main stream of spindle motors. The sensorless type motor doesn""t utilizes a sensor for detecting any rotary movement, for example a Hall-effect sensor.
The bipolar drive method is popularly used for driving such a motor as shown in FIG. 6 of the accompanying drawing. In FIG. 6, arrows (a) through (f) indicate the directions along which drive currents flow for the three phases of U, V and W that correspond to respective coils 30. With the bipolar drive method, an electric current is made to flow between two phases, e.g., from the U phase to the V phase. On the other hand, with the unipolar drive method, an electric current is made to flow only in a single phase. Thus, the bipolar drive method can generate a torque twice as strong as the torque generated by the unipolar drive method when an electric current of a same intensity is used. Therefore, with the bipolar drive method, it is possible to relatively reduce the number of wire windings of each coil of a motor.
A method of detecting the back-induced voltage (BIV) generated in each coil by the rotation of a magnet that is interlocked with the rotor of a sensorless type motor is normally used for detecting the position of the rotor. This method utilizes the fact that the back-induced voltage and the rotary position of the rotor show a constant relationship. Since this method makes the use of a Hall-effect sensor for detecting any rotary movement unnecessary, it is possible to produce a compact sensorless type motor at low cost.
FIG. 7 of the accompanying drawing shows a graph illustrating the relationship between the drive current (70) that flows in each phase and the back-induced voltage (71). The terminal voltage of each phase becomes a back-induced voltage only when no drive current flows. Therefore, in a sensorless type motor, the back-induced voltage is detected during a period when no electric current flows in any of the phases (72).
Meanwhile, disk drives are finding a broader scope of applications including not only memory devices of personal computers that have been taking a major area of applications but also data storage devices of various digital apparatus such as digital television sets. Such digital apparatus store data in the form of images and sounds, which may be read out from the disk drive comprised in the apparatus and reproduced from time to time. Then, the acoustic noise that the spindle motor of the disk drive emits when the disk drive is operated can give rise to a serious problem.
The acoustic noise problem can be solved by using a fluid dynamics bearing motor for the spindle motor because it can remarkably reduce the noise level. However, as the overall noise level is reduced by the use of a fluid dynamic bearing motor, the high frequency noise induced by electromagnetic force that is referred to as pure tone and occurs at the time of phase change during the operation of driving the motor can irritate people frequently. It is known that the pure tone is related to fluctuations in the torque of the rotor that occur when the motor is driven.
FIGS. 8A and 8B are graphs illustrating the torque of a motor when the motor is driven by means of the bipolar drive method, using a rectangular wave drive control technique of changing the drive current flowing in each phase of the motor into a rectangular current wave. As shown in FIG. 8B, the torque changes fiercely at each time of phase change (as indicated by an arrow). Therefore, a relatively large high frequency pure tone is generated with the bipolar drive method using a rectangular wave control technique.
FIGS. 9A and 9B are graphs illustrating the torque of a motor when the motor is driven by means of the bipolar drive method, using a trapezoidal wave drive control technique of changing the drive current flowing in each phase into a trapezoidal current wave. With the trapezoidal wave drive control technique, the torque changes mildly at each time of phase change (as indicated by an arrow in FIG. 9B) because this technique reduces the gradient of the drive current (to be referred to as slew rate thereinafter). Therefore, the high frequency pure tone is relatively reduced.
As described above, it is possible to reduce the pure tone during the motor is driven by using the trapezoidal wave drive control technique. However, trapezoidal wave drive control is less efficient than rectangular wave drive control and hence consumes power at a rate greater than the latter. In short, the reduction of pure tone and the increase of power consumption rate provide the problem of tradeoff.
Meanwhile, fluctuations in the torque of a motor driven by using sinusoidal wave drive control of changing the drive current flowing in each phase into a sinusoidal current are substantially reduced to nil as shown in FIGS. 10A and 10B. However, this technique is less efficient than trapezoidal wave drive control and consumes power at a rate higher than rectangular wave drive control by about 10%. Additionally, it is too difficult to apply the sinusoidal wave drive control technique to a sensorless type spindle motor because the drive current flows constantly in each phase and hence it is no longer possible to detect the back-induced voltage.
As described above, while the noise level of a spindle motor can be reduced by using a fluid dynamic bearing motor for it, it is necessary to reduce the pure tone that is generated at the time of phase change that takes place while the motor is being driven. While the pure tone can be reduced by using trapezoidal wave drive control of changing the waveform of the motor drive current into a trapezoidal waveform or sinusoidal wave drive control of changing the waveform of the motor drive current into a sinusoidal waveform, the both techniques are accompanied by a reduced efficiency and an increased power consumption rate if compared with the rectangular wave drive control technique.
Methods for driving a 3-phase brushless motor of the sensorless type that utilize a pulse width modulation (PWM) type drive control technique have been proposed (for example, inter alia, Jpn. Pat. Appln. KOKAI Publication No. 9-37584). These proposed methods are adapted to softer the rising edges of the drive current at the time of phase change by regulating the drive timing at the time of phase change. While these methods can reduce the pure tone, they cannot avoid the increase of the power consumption rate due to a reduced motor driving efficiency.
Methods for driving 3-phase brushless motors of the sensorless type that can be used with a sinusoidal wave drive control technique are also known (for example, inter alia, Jpn. Pat. Appln. KOKAI Publication No. 2000-278987). These known methods utilize a means for detecting the amplitude of the electric current in each phase to make it possible to use a sinusoidal wave drive control technique for a sensorless type motor and reduce the pure tone. However, with these methods, the phase of the back-induced voltage is determined by a CPU to force the latter to bear a heavy workload. Additionally, the CPU needs to know some constants of the brushless motor (including the reactance, the coil resistance and the torque constant) for the arithmetic operation of determining the phase of the back-induced voltage, taking possible variances of these values including those attributable to temperature changes into consideration. Therefore, these proposed methods may not be feasibly commercialized. Additionally, a sinusoidal wave drive control technique is accompanied by a relatively high power consumption rate as pointed out above but none of the proposed methods dissolve this problem.
Therefore, it is the object of the present invention to provide a disk drive adapted to selectively use a method of driving the spindle motor thereof where priority is given to noise suppression (reduction of pure tone) or a method of driving the spindle motor thereof where priority is given to reduction of the power consumption rate of the motor.
In accordance with one aspect of the present invention, there is provided a disk drive including an arrangement for controlling the spindle motor to match plural modes of operation. The plural modes of operation include a low power consumption rate mode where priority is given to reduction of the power consumption rate of the motor and a low noise mode where priority is given to noise suppression.
The disk drive comprises:
a disk for recording the data written by a head;
a spindle motor for rotating said disk;
a drive device for supplying a drive current to said spindle motor and driving it; and
a controller for controlling said drive device so as to make it drive said spindle motor by a drive current adapted to a low power consumption rate mode included in said plural modes or make it drive said spindle motor by a drive current adapted to a low noise mode also included in said plural modes.